In recent years, there has been intense interest concerning the grafting of polymers to inorganic surfaces. Such grafting can be accomplished by anionic, cationic or free radical processes. Among these processes, living (controlled) free radical polymerization (CFRP) has been shown to have an overwhelming advantage over other surface grafting methods in that it allows grafting of homo- and copolymers of controlled structure and molecular weight with a very high grafting density. Examples of all three major CFRP systems (ATRP, NMP and RAFT) have been successfully demonstrated using a “grafting from” technique for the preparation of organic-inorganic (O/I) nanocomposites.
The process by which the vast majority of O/I nanocomposites have been generated thus far utilizing CFRP methods and silica/silicate precursors usually involves two steps; first, a functionalization of the silica/silicate surface with a CFR agent by an ionic or covalent bond (remote from the attached CFR functional group). This reaction is most often conducted in bulk or in a solvent. Second, the CFR-functionalized silica/silicate is then suspended in a monomer (eg. styrene, acrylate etc.) in the presence of a free radical initiator. As the polymerization proceeds from the particle surfaces, the organic dispersability of the O/I composite improves dramatically. Unfortunately, such solution CFR polymerizations are typically very slow and give incomplete monomer conversion. This necessitates long reaction times and difficult monomer recovery.
Another widely used technique for the preparation of O/I nanocomposites utilizes the more rapid and environmentally friendly technique of emulsion polymerization and/or latex blending. Numerous variations on this method have been demonstrated. Typical of this method is the more-or-less conventional free radical polymerization of a monomer(s) in the presence of a highly dispersed surface-modified nano-size silica/silicate inorganic component. Alternatively, various techniques to combine preformed rubber latex (an aqueous dispersion of rubber nanoparticles) with aqueous dispersions of nanosized silica/silicates followed by coagulation will yield the nanocomposite. U.S. Pat. No. 6,759,464 and U.S. Patent Application 2004/0054059 describe some examples of technology in this area. While significant benefits can be obtained using these systems, control over the molecular weight, composition and polydispersity of the organic (polymer) component of the composite is lacking.
Therefore, the need still exists to develop practical methods whereby the best features of CFRP technology (e.g. control over the molecular weight, composition and polydispersity) can be combined with the simplicity of an emulsion process to yield useful nanocomposites materials.
U.S. Pat. No. 5,405,985, U.S. Pat. No. 5,468,893, U.S. Pat. No. 5,583,245, U.S. Pat. No. 5,663,396, and U.S. Pat. No. 6,172,251, as well as subsequent improvement patents (U.S. Pat. No. 6,680,398, U.S. Pat. No. 6,534,668; U.S. Pat. No. 6,448,426; U.S. Pat. No. 6,384,256, and U.S. Pat. No. 6,384,255) disclose the utilization of aqueous phase transfer technology for the preparation of sulfur-containing alkoxysilanes.